Cellulases are enzymes which hydrolyze cellulose (.beta.-1,4-D-glucan linkages) and produce as primary products glucose, cellobiose, cellooligosaccharides, and the like. Cellulases are produced by a number of microorganisms and comprise several different enzyme classifications including those identified as exo-cellobiohydrolases (CBH), endoglucanases (EG) and .beta.-glucosidases (BG) (Schulein, M, 1988 Methods in Enzymology 160: 235-242). Moreover, the enzymes within these classifications can be separated into individual components. For example, the cellulase produced by the filamentous fungus, Trichoderma longibrachiatum, hereafter T.longibrachiatum, consists of at least two CBH components, i.e., CBHI and CBHII, and at least four EG components, i.e., EGI, EGII, EGIII and EGV (Saloheimo, A. et al 1993 in Proceedings of the second TRICEL symposium on Trichoderma reesei Cellulases and Other Hydrolases, Espoo, Finland, ed by P. Suominen & T. Reinikainen. Foundation for Biotechnical and Industrial Fermentation Research 8: 139-146) components, and at least one .beta.-glucosidase. The genes encoding these components are namely cbh1, cbh2, egl1, egl2, egl3, and egl5 respectively.
The complete cellulase system comprising CBH, EG and BG components synergistically act to convert crystalline cellulose to glucose. The two exo-cellobiohyrolases and the four presently known endoglucanases act together to hydrolyze cellulose to small cello-oligosaccharides. The oligosaccharides (mainly cellobioses) are subsequently hydrolyzed to glucose by a major .beta.-glucosidase (with possible additional hydrolysis from minor .beta.-glucosidase components).
Protein analysis of the cellobiohydrolases (CBHI and CBHII) and major endoglucanases (EGI and EGII) of T. longibrachiatum have shown that a bifunctional organization exists in the form of a catalytic core domain and a smaller cellulose binding domain separated by a linker or flexible hinge stretch of amino acids rich in proline and hydroxyamino acids. Genes for the two cellobiohydrolases, CBHI and CBHII (Shoemaker, S et al 1983 Bio/Technology 1, 691-696, Teeri, T et al 1983, Bio/Technology 1, 696-699 and Teeri, T. et al, 1987, Gene 51, 43-52) and two major endoglucansases, EGI and EGII (Penttila, M. et al 1986, Gene 45, 253-263, Van Arsdell, J. N. et al 1987 Bio/Technology 5, 60-64 and Saloheimo, M. et al 1988, Gene 63, 11-21) have been isolated from T. longibrachiatum and the protein domain structure has been confirmed.
A similar bifunctional organization of cellulase enzymes is found in bacterial cellulases. The cellulose binding domain (CBD) and catalytic core of Cellulomonas fimi endoglucanase A (C. fimi Cen A) has been studied extensively (Ong E. et al 1989, Trends Biotechnol. 7:239-243, Pilz et al 1990, Biochem J. 271:277-280 and Warren et al 1987, Proteins 1:335-341). Gene fragments encoding the CBD and the CBD with the linker have been cloned, expressed in E. coli and shown to possess novel activities on cellulose fibers (Gilkes, N. R. et al 1991, Microbiol Rev. 55:305-315 and Din, N et al 1991, Bio/Technology 9:1096-1099). For example, isolated CBD from C. fimi Cen A genetically expressed in E. coli disrupts the structure of cellulose fibers and releases small particles but have no detectable hydrolytic activity. CBD further possess a wide application in protein purification and enzyme immobilization. On the other hand, the catalytic domain of C. fimi Cen A isolated from protease cleaved cellulase does not disrupt the fibril structure of cellulose and instead smooths the surface of the fiber.
These novel activities have potential uses in textile, food and animal feed, detergents and the pulp and paper industries. However, for industrial application, highly efficient expression systems must be procured that produce higher yields of truncated cellulase proteins than are currently available to be of any commercial value. For example, Trichoderma longibrachiatum CBHI core domains have been separated proteolytically and purified but only milligram quantities are isolated by this biochemical procedure (Offord D., et al 1991, Applied Biochem. and Biotech. 28/29:377-386). Similar studies were done in an analysis of the core and binding domains of CBHI, CBHII, EGI and EGII isolated from T. longibrachiatum after biochemical proteolysis, however, only enough protein was recovered for structural and functional analysis (Tomme, P et al, 1988, Eur. J. Biochem 170:575-581 and Ajo, S, 1991 FEBS 291:45-49).
In order to obtain strains which express higher levels of truncated cellulase proteins than previously realized, applicants chose T. longibrachiatum as the microorganism most preferred for expression since it is well known for its capacity to secrete whole cellulases in large quantities. Thus, applicants set out to genetically engineer strains of the above filamentous fungus to express high levels of bioengineered novel protein truncated cellulases.
It remained unknown before Applicants invention whether the DNA encoding truncated cellulase binding and core domain proteins could be transformed into Trichoderma in such a manner as to overexpress novel truncated cellulase genes into functional proteins without deterioration in the host cell and obtained secretion to facilitate identification and purification of the engineered product. Recently, Nakari and Penttila have shown that it is possible to genetically engineer a Trichoderma host to express a truncated form of the Trichoderma EGI cellulase, specifically the catalytic core domain, however the level of expression of EGI core domain was low (Nakari, T. et al, Abstract P1/63 1st European Conference on Fungal Genetics, Nottingham, England, Aug. 20-23, 1992). Moreover, it was unknown whether a Trichoderma cellobiohydrolase catalytic core domain or any Trichoderma cellobiohydrolase or endoglucanase cellulose binding domain could be produced by recombinant genetic methods.
Accordingly, it is an object of the present invention to introduce DNA gene fragments into strains of the fungus, Trichoderma longibrachiatum to produce transformant strains that express high levels of novel truncated protein (grams/liter level) engineered cellulases from the binding and core domains of Trichoderma cellulases. The truncated proteins are correctly processed and secreted extracellularly in an active form. The present invention further relates to the novel truncated proteins isolated from these transformants.